This invention relates to the production of aluminum chloride from clay. More particularly, this invention relates to an improved process for the chlorination of clays containing silicon oxide as well as aluminum oxide whereby the aluminum oxide is chlorinated while the chlorination of the silicon oxide is suppressed.
In the production of aluminum chloride from clay by the chlorination of the aluminum oxide in the clay, undesirable amounts of silicon chloride are produced by the concurrent chlorination of the silicon oxide within the clay. It is known to suppress the chlorination of the silicon oxide by adding silicon chloride to the chlorinating agent. For example, Staib U.S. Pat. No. 1,866,731 teaches suppressing of the formation of silicon chloride in clay by mixing equal parts of silicon chloride and chlorine together. Staib states that the silicon chloride may be obtained from the condensation of the resultant chlorination product of the clay, i.e. by recycling the silicon tetrachloride separated from the aluminum chloride produced in the chlorination reaction.
It would be, in fact, desirable to recycle back to the reaction all of the silicon chloride recovered from the chlorinated clay mixture of aluminum chloride and silicon chloride to minimize the disposal of silicon chloride which is of minimum economic value. However, it has been discovered that the recycling back of large amounts of silicon chloride while indeed suppressing the chlorination of silicon oxide in the clay, adds considerably to the reaction costs by adding the cost of separation as well as the recycle cost. Thus, it would be desirable to minimize the amount of silicon chloride produced. It also has been found that the addition of large amounts of silicon chloride to the clay chlorination reaction, while suppressing the chlorination of the silicon oxide, may also interfere with the chlorination of aluminum. For example, D. J. Milne in an article entitled "Chlorination of Bauxite in the Presence of Silicon Tetrachloride" in Metallurgical Transactions, Vol. 6B, September 1975 teaches that the addition of silicon chloride to the Cl.sub.2 stream markedly reduced both the alumina chlorination rate and the degree of conversion.
It is also known that the addition of an alkali metal halide such as sodium chloride or potassium chloride has a beneficial effect on the chlorination of aluminum oxide contained in kaolin. For example, A. E. Seferovich in "Preparation of Anhydrous AlCl.sub.3 by Kaolin Chlorination Using a Catalyst" Journal of Chemical Industry (Moscow) Vol. 10, 1934, pages 62-64 discusses the use of KCl, NaCl, and many other metal chlorides and their possible function as catalysts in the chlorination reaction. It has been postulated by others that the use of KCl may accelerate the chlorination of the clay due to the fact that the KCl forms a binary salt of low volatility with Al.sub.2 Cl.sub.6 which has low vapor pressure and which means that carrying this component out of the reaction zone shifts the reaction in favor of the formation of Al.sub.2 Cl.sub.6. (See Voronon and Galinker in "Production of Anhydrous Aluminum Chloride from Chasov-Yar Clay", Journal of Chemical Industry [Moscow] Vol. 7, 1930, pages 143-149.)
However, we have found nothing in the literature to indicate what effect the use of a catalyst has on the chlorination of the silicon oxide present in the kaolin to form silicon chloride. Since, as previously mentioned, it is desirable to recycle back to the reaction all of the silicon chloride made during the chlorination and further since the recycling back of large amounts of silicon chloride is undesirable from a standpoint both of process economics and the possible suppression of the aluminum oxide chlorination reaction, it would appear to be undesirable to use a catalyst in the reaction from the standpoint of the coproduction of undesirably excessive amounts of silicon chloride.
Quite surprisingly, however, it has been found that the use of a catalyst in the chlorination of clays such as kaolin containing both alumina and silica in the presence of chlorine gas or phosgene and a reducing agent such as carbon monoxide or carbon, while recycling back to the reaction zone any silicon chloride produced therein results in an enhanced yield of aluminum chloride from the aluminum oxide present in the clay while reducing the amount of silicon chloride produced in the chlorination reaction.
While we do not wish to be bound by any theory, it would appear that possibly the addition of a catalyst provides a catalytic effect on the chlorination of aluminum oxide by silicon chloride in accordance with the following reaction equation: ##STR1##